Teachers, social workers and nurses would be forced to pay thousands of pounds more for their university degrees if ministers replaced tuition fees with a graduate tax, according to a report today.

The University and College Union (UCU) findings suggest that relatively low earners, such as nurses and teachers, would pay thousands of pounds more if a graduate tax was introduced.

Vince Cable, the business secretary, last month floated the idea of a tax tied to earnings, through which low earners would pay less than they now do for their degrees while those with high incomes paid more. He said low earners were being treated unfairly.

"It surely can't be right that a teacher or care worker or research scientist is expected to pay the same graduate contribution as a top commercial lawyer or surgeon or City analyst whose graduate premium is so much bigger," he said in a speech to universities.

Senior Conservative sources at Number 10 were reported to oppose a graduate tax in favour of higher tuition fees than the current £3,225 a year.

The universities minister, David Willetts, yesterday did not endorse a graduate tax directly, but said he favoured a graduate contribution. He is understood to have concerns that a badly designed graduate tax could see high earners paying far more than the cost of their university education in tax, so leading them to pursue their university career abroad.

Asked on the BBC's Andrew Marr Show whether the government supported a graduate tax or contribution, he said: "We do have a preference for graduates, after they have got into work … making a higher contribution to the benefits of having a university education. That seems to be a sensible approach."

An independent review, led by the former BP chief executive Lord Browne, is looking into alternatives to tuition fees and will report its findings this autumn.

Willetts said: "We have said to Lord Browne: 'Look at a graduate contribution.' We think it is a progressive tax. It is very important that every option is part of the review and is looked into. But there are other key criteria that will be looked into, such as social mobility in higher education and the need for a more diverse range of higher education providers."

UCU's report found that teachers would be forced to pay £17,271 more than they do now to cover the cost of their degrees under a graduate tax. That is almost half the annual salary they average across their working lives – £36,837.

Nurses and social workers would have to pay an extra £7,824 and £8,528 respectively for their courses under a graduate tax, more than one-quarter of the annual salaries they average across their working lives — £29,494 and £30,040 respectively.

Doctors would end up paying £77,118 more than they do now, compared to an average annual salary of £84,451, while public sector workers would have to pay back an extra £9,649, almost one-third of their average salary of £30,900.

The calculations were made according to a graduate tax of 5% on earnings over £15,000. It assumed that graduates would pay the tax for no more than 25 years of their working lives. Under the current system of tuition fees, graduates pay 9% of all their income over £15,000. Everyone pays the same rate, but the more you earn, the quicker it is cleared.

Sally Hunt, the UCU general secretary, said parents and students would judge changes to finance on whether they made university more expensive. She said: "Whatever scheme is proposed, the government must ensure that studying for key professions remains attractive and that the prospect of prohibitive costs over a lifetime will not put off the next generation of innovators and public servants."

A spokesman for the Department for Business, Innovation and Skills said: "The government has asked Lord Browne to look at a variable graduate contribution amongst the options he is considering to reform the funding of universities. We will need to see what options and proposals are put forward by the review this autumn.

"While it is good to have wide public debate about future funding options, it is also important to be clear that the current system is no longer fit for purpose. We need a new funding settlement which promotes world-class competitiveness in teaching and research, with better quality for students."